Adhering substance detector and controller using the same

An object sensor is provided, which has a high object detection rate with respect to an object such as a raindrop adhering to a detection surface, and which is capable of detecting the presence/absence of an object with a high sensitivity. A light source 10, a detection surface 110, a focusing lens 40, and a plurality of photodetecting elements of a photodetecting unit 50 form a focusing system of equal magnification that is capable of catching an image in a size smaller than an object to be detected. Light emitted from the light source 10 is incident on the detection surface 110 via a prism 20 at a predetermined incident angle. Without a raindrop 120, the total internal reflection condition is satisfied as shown by a path 140 of a beam, so that the beam is subjected to the total internal reflection. Then, the beam passes through a prism 30, and is focused on a photodetecting surface of the photodetecting unit 50 by the focusing lens 40. On the other hand, with a raindrop 120, the total internal reflection condition is not satisfied as shown by a trail 130 of a beam, so that the beam passes therethrough and is not focused on the photodetecting unit 50. A difference in signal level is determined on the photodetecting element basis, so that an object covering the detection surface 110 is detected.